This invention relates to a noise-suppressing high-tension resistance cable (hereinafter referred to as an "ignition cable") for suppressing noise, produced by an electronic ignition of an internal combustion engine of an automobile or the like, which propagates through the air via the cable.
In a conventional ignition cable, in order to prevent such electromagnetic noise wave troubles, and also to prevent an undesired voltage drop from developing when the cable is subjected to water, it has been required that the resistivity of the conductor be about 16 k.OMEGA./m, and that the capacitance be not more than 80 pF/m. The overall outer diameter of an ignition cable having such resistivity and capacitance is usually is 7 mm or 8 mm.
Japanese Patent Application Unexamined Publication No. 107410/81 discloses a cable shown in FIG. 6, which meets the above requirements. In this cable, the outer diameter of a resistance conductor a is not more than 1.2 mm. A semi-conductive layer composed of an inner semi-conductive layer c, a separation layer d, and an outer semi-conductive layer e is formed around a tension member b composed of an aramid fiber bundle. An insulator layer f outside the semi-conductive layer is made of crosslinked polyethylene or a crosslinked blend containing polyethylene. With this construction, the capacitance is not more than 80 pF/m. A reinforcement layer g and a protective sheath layer h are disposed, in that order, around the insulator layer f.
The conventional ignition cable shown in FIG. 6 meets the requirement that the capacitance be not more than 80 pF/m, since the outer diameter of the cable is 7 mm or 8 mm. However, such an ignition cable with an outer diameter of not more than 5 mm which has been developed to meet recent lightweight and small-diameter requirements does not meet the capacitance requirement.
Further, noise suppression regulations for automobiles in Europe and other countries have become more strict, and sufficient noise suppression effect cannot be achieved merely by forming the inner and outer semiconductive layers c and e by a solid method or a carbon coating dipping method.
In connection with the method of forming the resistance conductor a, it has been required that the resistivity be less varied by a high temperature atmosphere, a thermal cycle during the actual running of the automobile, and physical variations such as vibration and bending.